Conventionally, and particularly in mass transit braking equipment, an emergency brake application is initiated electrically, that is, the emergency vent valve devices on each of the cars making up a train, are actuated simultaneously by energization of a continuous electrical circuit running throughout the train. A piston in the emergency vent valve is subjected on opposite sides, through a choke, to brake pipe pressure so that when the pressure on one side of the piston is suddenly, or at an emergency rate, reduced by said electrical actuation, the differential created across the piston causes the piston to open the vent valve for unrestrictedly opening the brake pipe to atmosphere and thereby propogating an emergency brake application. This type of vent valve device is effective as long as brake pipe pressure on opposite sides of the piston is sufficient to provide a pressure differential thereacross when pressure on one side is released. It is possible, however, that pressure may leak from the brake pipe at such a slow rate as to go undetected until brake pipe pressure reaches such a low level as to be ineffective for establishing a pressure differential across the piston upon venting of pressure on one side thereof, thereby rendering the apparatus ineffective for an emergency brake application. Moreover, due to such slow and undetectable leakage, pressure in the brake pipe may reach a low pressure level below that at which any braking action, either service or emergency, could be effected.